1. Field of the Invention
The present invention relates to a radiation therapy apparatus used for a therapy of diseases such as malignant tumors, and particularly to the radiation therapy apparatus including a multi-leaf collimator device which allows an extent of an object that is exposed to radiation (which will be referred to as an “irradiation field” hereafter) to be set with high precision.
2. Description of the Related Art
From the perspective of radiation protection, the radiation therapy apparatus includes a collimator device formed of a material the nature of which renders it impermeable to radiation such as tungsten or the like, thereby allowing the exposure to radiation to be limited to a therapy part including a body. Such a collimator device needs to have a function of carefully forming the irradiation field that approximates a shape of the therapy part without a formation of a penumbra. Accordingly, such a collimator device has a first collimator and a second collimator arranged in the irradiation direction such that they overlap.
With such an arrangement, the first collimator provided on a near side of a radiation source is configured in a form of a single unit comprising a pair of members disposed such that they face each other across an irradiation axis. Such an arrangement allows that the members drive so as to adjust a distance therebetween. For example, the members drive along an arc-shaped path around the radiation source as a center. On the other hand, the second collimator provided on a far side of the radiation source is configured in the form of a pair of collimator components (blocks) such that the collimator components face each other across the irradiation axis in an orthogonal direction to a moving direction of the first collimator. Each of the collimator components of the second collimator has multiple leaves arranged close to one another, which can be individually moved so as to adjust a distance therebetween along the arc-shaped path around the radiation source as the center.
The second collimator is the so-called multi-leaf collimator device comprising a pair of collimator components 1A and 1B arranged symmetrically, as shown in FIG. 8, for example. The collimator component 1A has tens of leaves 1A1-1An arranged close to one another. The collimator component 1B has tens of leaves 1B1-1Bn arranged close to one another. The leaves 1A1-1An and the leaves 1B1-1Bn can be adjusted individually along the arc-shaped path. Such an arrangement allows these leaves to be driven individually along the arc-shaped pat by respective driving devices each of which is provided to the corresponding leaf. Such an arrangement allows the first collimator comprising the collimator components facing each other to be moved in an X direction so as to adjust the distance therebetween. In addition, such an arrangement allows the leaves 1A1-1An and the leaves 1B1-1Bn facing one another to be moved individually in a Y direction so as to adjust the distance therebetween. Such a combination of adjustment operations provides a formation of an irradiation field “U” in a desired shape that approximates the shape of the therapy part. Such an arrangement allows the leaves facing one another to be moved along the arc-shaped path, thereby providing irradiation without the formation of the penumbra in the irradiation field “U”.
It is extremely important for the radiation therapy apparatus to irradiate only the therapy part which is approximately equal to a focus without exposing a healthy tissue to radiation. From this point of view, the multi-leaf collimator provides an extremely important function. However, the focus develops in various shapes. This leads to difficulty in providing the irradiation field “U” that matches the shape of such a focus. In order to solve this problem, examples of conceivable arrangements include an arrangement in which the collimator components 1A and 1B include the leaves 1A1-1An and 1B1-1Bn, respectively, which are a minute pitch. Such an arrangement requires as many leaves A1-1An and 1B1-1Bn as possible, by narrowing a leaf-width.
However, such an arrangement having an increased number of leaves, which provides the minute pitch, requires leaves with a reduced the leaf-width. Even with ordinary arrangements, the leaves are formed with the leaf-width of around 2 to 3 [mm]. An arrangement having leaves with a narrower width than those of ordinary leaves has problems in the manufacturing process due to warped leaves etc., examples of which include difficulty in manufacturing the collimator with a desired level of precision.
In addition, these leaves must be moved individually so as to provide the desired irradiation field. Such an arrangement requires respective driving device. This leads to difficulty in designing a layout of a supporting mechanism and a driving mechanism, which allow such a great number of leaves arranged closely adjacent to one another to be individually moved, and the layout of a detection mechanism for detecting a movement of each leaf. Furthermore, in order to prevent radiation leaks from gaps between the adjacent leaves, there is a need to arrange the leaves with as small the gap as possible between the adjacent leaves, e.g., with the gap of around 0.05 to 0.1 [mm]. However, it is extremely difficult to provide such high-precision manufacturing and fabrication in order to realize such a design.
In view of such a situation, an arrangement has been proposed in which the second collimator is provided in a form of two separate stages arranged with a predetermined interval along an irradiation direction. With such an arrangement, the leaves are arranged with an offset between an upper-stage collimator and a lower-stage collimator such that each boundary between the adjacent leaves including upper-stage collimator components respectively does not match any of the boundaries between the adjacent leaves of the lower-stage collimator components respectively (see “Japanese Patent Publication (Laid-open: KOKAI) No. 2002-210026”, for example). With such an arrangement, although the leaves are formed with the same leaf-width as those of the ordinary arrangements, such an arrangement provides the same effect as that provided by an arrangement having leaves formed with half the leaf-width of those of the ordinary arrangements. This is able to make the irradiation field further minute. Such an arrangement allows the shape of the irradiation field to be adjusted with higher precision such that it approximates the therapy part, thereby protecting the healthy tissue from being exposed to radiation.
Such a technique disclosed in the aforementioned Japanese Patent publication can eliminate to a certain extent the problem of an arrangement having an increased number of leaves formed with a reduced leaf-width. However, such a technique provides an arrangement in which the second collimator is provided in the form of two separate stages arranged with an interval in the irradiation direction. Accordingly, the number of components required for the supporting mechanism and the driving mechanism, which allow the leaves to be individually moved, and the number of components required for the detection mechanism for detecting the movement of each leaf, are twice those of the ordinary arrangements. This leads to a complicated configuration and a larger-sized collimator device, which are new problems. Such a larger-sized collimator leads to reduction in a space for the therapy, which is a problem of the radiation therapy apparatus itself.